Keiji Misawa

Demonstration of REE fractionation among individual chondrules from the Allende (CV3) chondrite

Abstract Abundances of REE, Ba, Sr, Rb, K, Mg and Ca were determined by precise mass spectrometric isotope dilution techniques for 24 chondrules from the Allende (CV3) chondrite. The REE abundances are 2.5−10 × CI for barred olivine chondrules, 2−8 × CI for porphyritic and nonporphyritic pyroxene chondrules, 0.15−4 × CI for porphyritic olivine and porphyritic olivine-pyroxene chondrules and are more or less similarly fractionated. General REE fractionations and large (up to 170%) anomalies of Ce, Eu and Yb occur in all chondrule types, particularly for barred olivine and pyroxene-rich chondrules. Positive correlations of REE with the moderately volatile elements, K and Rb, as well as other refractory elements, Ca, Sr and Ba, are independent of textural type and major chemical compositions. Each type of chondrule has large and systematic abundance variations of K and Rb, but shows a constant K Rb ratio close to that of CIs. From these results, the following constraints on the chemical characteristics of precursors and chondrule-forming events are suggested: (1) vaporization loss of alkalis accompanied by K Rb fractionation did not occur during chondrule-formation melting events, (2) elemental abundances were basically established prior to melting events by accretion of alkali-free components) and alkali-bearing refractory precursors with fractionated REE, (3) gas/solid (or liquid) processes yielding REE fractionations took place during the formation of refractory precursors.

Highly fractionated REE in the Hedjaz (L) chondrite: implications for nebular and planetary processes

Abundances of REE, Sr, Rb, K, Ca and Mg in two whole-rock fragments, two lithic fragments and three chondrules of the Hedjaz (L3) chondrite were determined by isotope dilution mass spectrometry. One whole-rock fragment shows a step pattern with the light-REE enriched, indicating that the meteorite contains components with highly fractionated REE abundances. The chondrules (PP, BO and one unknown type) show variable (0.8 ∼ 2 × CI) REE abundances and almost flat REE patterns with minor irregularities of Ce, Eu and Yb. n nAnomalous REE patterns were identified for two light-colored lithic fragments. A pyroxene-rich, glass-bearing subrounded clast I has low REE abundances (0.3 ∼ 0.8 × CI) together with depletion of other lithophiles (Ca, Al, Sr, Na and K). It shows a light-REE enhanced pattern with positive anomalies of Ce, Eu and Yb. This is a new REE pattern reported for lithic clasts from ordinary chondrites. It is suggested that the clast formed through melting processes (possibly under planetary conditions) from the condensates either from a later stage nebular gas or from gas vaporized from dust. n nThe second clast II (Ca = 1.1 ∼ 1.9%) consists mainly of coarse-grained olivine, minor pyroxene and plagioclase, and trace amounts of glass; it shows an igneous texture with shock features. It has a strong positive correlation of plagiophile elements (Ca, Sr and Eu) but heterogeneous distributions of common REE from portion to portion. One of the chips indicates a remarkable REE fractionation (LREE = ∼ 40 ×,HREE = ∼ 1.7 × CI) similar to that of Group II CAIs of carbonaceous chondrites. This is the first identification of Group II REE pattern in lithic clasts from ordinary chondrites. It is suggested that the clast had formed by shock-induced melting of an inhomogeneous CV-like precursor assemblage carrying high-temperature nebular REE components on a grand-parent body and was then incorporated into the Hedjaz meteorite parent body. n nThe presence of impact-related products with highly fractionated REE components in a chondritic breccia indicates the existence of related physico-chemical conditions in the formation of chondrules, clasts and their precursors with that of CAIs.

Noble Metal Enrichments in Cosmic Spherules

In this work, studies on relationships of chemical compositions bexad tween fusion crust and nucleus in iron spherules are reported. More than 10% of the iron spherules which were picked out from deep sea sediment, have cores and crusts. We were able to divide three of them into cores and crusts. Each cores and crusts were analyzed individually by INAA. The core mainly consists of iron and nickel. Other trace elements, especially noble metal Au and Ir were concenxad trated in the core. The mechanism of core formation in the iron spherules shows us the origin of them. ABSTRACT. In this work, studies on relationships of chemical compositions bexad tween fusion crust and nucleus in iron spherules are reported. More than 10% of the iron spherules which were picked out from deep sea sediment, have cores and crusts. We were able to divide three of them into cores and crusts. Each cores and crusts were analyzed individually by INAA. The core mainly consists of iron and nickel. Other trace elements, especially noble metal Au and Ir were concenxad trated in the core. The mechanism of core formation in the iron spherules shows us the origin of them.

Redistribution of Radiogenic Lead in Plagioclase during Shock Metamorphism

It is well known that there was heavy meteoritic bombardment of the lunar surface around 3.9 Ga (lunar terminal cataclysm [1]), not only resulting in excavation of crustal rocks but also triggering mare basalt volcanism. Plagioclase is the main constituent of the lunar crust, which is considered a product of a primordial Moon-wide “magma ocean” [2], an outer layer that was partially molten to a depth of several hundred kilometers. Because of the key role of plagioclase in the U-Th-Pb systematics of lunar highland rocks and mare basalts, it is important to understand the mobility of volatile lead in plagioclase during shock metamorphism. Almost all of the samples recovered from the Apollo and Luna missions have experienced shock metamorphism. With increasing shock intensity, plagioclase converts to the isotropic glass, “maskelynite,” which is ubiquitously observed in highland rocks and mare basalts.